Method for forming thin mgf2 film and low-reflection film

ABSTRACT

Disclosed herein is a method for forming a thin MgF 2  film on a substrate which comprises coating a substrate with a liquid containing an Mg salt and a BF 3  complex salt and subsequently heating the coating, and the method may be used to coat a substrate such as glass plate, optical parts, CRT panel and liquid crystal display panel with a low-reflection film composed of a transparent thin MgF 2  film having a low refractive index and good durability.

TECHNICAL FIELD

The present invention relates to a method for forming on a substrate atransparent thin MgF2 film having a low refractive index and also to amethod for forming a low-reflection film on a substrate.

BACKGROUND ART

There have been proposed many methods for forming a low-reflection filmon a substrate in the field of cathode-ray tube for TV and computerterminals, not to mention sheet glass, optical parts and opticalinstruments.

Conventional methods consist of coating the surface of a cathode-raytube with an SiO₂ layer having minute irregularities, thereby producingthe anti-glare effect, or consist of etching the surface of acathode-ray tube with hydrofluoric acid, thereby forming surfaceirregularities, as disclosed in Japanese Unexamined Patent PublicationNo. 118931/1986. The coating and etching by these conventional methodsare called non-glare treatment because they merely form minuteirregularities which reflect and scatter the incident light; they arenot essentially designed to form a low-reflection film and hence theyhave a limit in decreasing the reflectivity of the substrate.

On the other hand, attempts have been made to coat the surface of lensor glass with a thin film of MgF₂ (which is a stable substance having alow refractive index) by vacuum deposition. Unfortunately, vacuumdeposition has a disadvantage of requiring an expensive apparatus andinvolving difficulties in handling large objects (such as finishedcathode ray tube and sheet glass) and a large number of objects for massproduction. All this leads vacuum deposition to high production cost.

The present invention was completed to eliminate the above-mentioneddisadvantages involved in the conventional technology. Accordingly, itis an object of the present invention to provide a new method forforming on a substrate a stable, transparent thin film having a lowrefractive index in a simple chemical manner. It is another object ofthe present invention to provide a new method for forming on a substratea thin film having the low-reflection characteristics.

DISCLOSURE OF THE INVENTION

The first aspect of the present invention resides in a method forforming a thin MgF₂ film on a substrate which comprises coating asubstrate with a liquid containing an Mg salt and a BF₃ complex salt andsubsequently heating the coating.

The second aspect of the present invention resides in a method forforming on a substrate a low-reflection film of monolayer or multilayerstructure, with at least one layer being a thin MgF₂ film, said processcomprising coating a substrate with a liquid containing an Mg salt and aBF₃ complex salt and subsequently heating the coating, thereby formingsaid thin MgF₂ film.

The third aspect of the present invention resides in a method forforming on a substrate a thin MgF₂ film and low-reflection film whichcomprises coating a substrate with a solution containing an MgF₂ solformed by the reaction between an Mg salt and a BF₃ complex salt, andsubsequently heating the coating.

The fourth aspect of the present invention resides in a method forforming on a substrate a thin MgF₂ film and low-reflection film whichcomprises coating a substrate with a solution containing a siliconcompound and an MgF₂ sol formed by the reaction between an Mg salt and aBF₃ complex salt, and subsequently heating the coat.

In the preferred embodiments of the present invention, the Mg salt isMgX₂ (X=a halogen element other than fluorine), and is used incombination with a BF₃ complex salt.

According to the method of the present invention, the thin MgF₂ film isformed from an Mg salt (excluding a fluoride) and a BF₃ complex saltwhich functions as a fluorinating agent. The reaction to form the thinMgF₂ film is promoted if the Mg salt and BF₃ complex salt as thestarting materials are dispersed, mixed or dissolved in a solution, andthe reaction rate is increased by heating.

The Mg salt as the starting material may be available in various formssuch as those represented by the formula MgX₂ (where X denotes a halogenexcluding fluorine). Preferred examples of the Mg salt include MgCl₂,MgBr₂ and MgI₂. Among these with MgCl₂ being most desirable. They may bein the form of either anhydride or hydrate. Other preferred examples ofthe Mg salt include hydroxide, carbonate, sulfate, nitrate, perchlorateand acetate. Additional preferred examples of the Mg salt include Mgalkoxides represented by the formula Mg(OR)₂ (where R denotes an alkylgroup), organic acid salts including carboxylates (such as acetate,formate, oxalate and benzoate), and acetylacetone complex.

The BF₃ as the other starting material may be available in variousforms. Preferred examples include BF₃ alkyl ether complex salt, BF₃phenol complex salt, BF₃ alcohol complex salt and BF₃ aqueous solutioncomplex salt. Typical examples include ethyl ether complex salt,methanol complex salt, ethanol complex salt, acetic acid complex saltand phenol complex salt. The solvent is not specifically limited, but itincludes water, aqueous solution, alcohol, ester, ether and ahigh-dielectric organic solvent (such as propylene carbonate andγ-butyrolactone). Preferred solvents are alcohols, especially methanol,ethanol, propanol and butanol, which keep the starting materials stable.

The Mg salt and BF₃ complex salt should be used in a molar ratio of 1:2to 4:1, especially 1:1 to 2:1. They should be dissolved in the solventat a concentration of 1-30 wt %.

The solution from which the thin MgF₂ film is formed according to themethod of the present invention may be prepared at room temperature.However, it is possible to accelerate the reaction by heating thesolution up to a temperature below the boiling point of the solvent(e.g., up to about 100° C. in the case of alcohol). Time required forthe reaction to form the MgF₂ sol ranges from 10 minutes to 6 hours,depending on the heating temperature. (For example, it is about 1 hourat the heating temperature of 85° C.)

It is possible to produce a solution which contains an MgF₂ sol having adesired particle diameter and particle size distribution, if Mg salt,BF₃ complex salt, concentration, solvent, heating temperature andreaction time are properly selected.

The solution prepared as mentioned above may be applied to a substrate.However, the solution may be filtered to remove the starting materialswhich remain undissolved. The solution may also be filtered to separatethe MgF₂ powder which has been formed by the reaction, if it isnecessary to mix the MgF₂ powder with another substance. In this case,the MgF₂ powder may be peptized again in a proper solvent using a ballmill, sand mill, homomixer or stirrer.

According to the present invention, the starting materials to form athin MgF₂ film or the solution or sol containing MgF₂ may beincorporated with an additive, binder or filler, which increases theadhesion strength and hardness of the thin MgF₂ film or improves thestability of the solution or sol. Examples of the additive includeSi(OR)₄ (R=alkyl group), Si(OR)_(x) ·R_(x-4) (x=3 or 4, R=alkyl group),and partial hydrolyzate thereof, which cause SiO₂ to separate out duringthe reaction. Examples of the binder include, in addition to theabove-mentioned silicon compounds, Zr(OR)₄ (R=alkyl group), Ti(OR)₄(where R=alkyl group), Al(OR)₃ (R=alkyl group), Zr(C₅ H₇ O₂)_(n)·(OR)_(m) (n=1 to 4, m=0 to 3, n+m=4, R=alkyl group), Ti(C₅ H₇ O₂)_(n)(OR)_(m) (n=1 to 4, m=0 to 3, n+m= 4, R=alkyl group), Al(C₅ H₇ O₂)_(n)·(OR)_(m) (n=1 to 3, m=0 to 2, n+m=3, R=alkyl group), Zr(OR)_(x)·R_(4-x) (x= 1 to 3, R=alkyl group), Ti(OR)_(x) ·R_(4-x) (x= 1 to 3,R=alkyl group), Al(OR)_(x) R_(3-x) (x=1 or 2, R=alkyl group), andpartial hydrolyzate thereof, which cause ZrO₂, TiO₂ and Al₂ O₃ toseparate out individually or in the form of mixture or complex togetherwith MgF₂, MgF₂ and SiO₂. Moreover, the solution may be incorporatedwith a proper surface active agent to improve its ability to wet thesubstrate. Examples of the surface active agent include sodium linearalkylbenzenesulfonate and alkyl ether sulfate ester.

For the thin MgF₂ film to have electrical conductivity, it is necessaryto incorporate an electroconductive metal compound into the solutioncontaining an Mg salt and a BF₃ complex salt (and an optional compoundwhich forms at least one kind of SiO₂, ZrO₂, TiO₂ and Al₂ O₃). Examplesof the metal compound include the organic salts (such as acetylacetonateand alkoxide) and inorganic salts (such as halide, acetate, nitrate andchelate) of metals (such as Sn and In) which form electricallyconductive metal oxides (such as SnO₂ and In₂ O₃ containing Sn (ITO)).These metal compounds give rise to an electrically conductive metaloxide such as SnO₂ and Sn-containing In₂ O₃ (ITO) simultaneously withthe separation of MgF₂, and MgF₂ other oxides.

The metal compound to impart electrical conductivity may be replaced bya separately prepared colloid solution containing fine powder ofelectrically conductive oxide such as Sb-SnO₂ (Sb-doped SnO₂), SnO₂,F-doped SnO₂ or ITO.

According to the present invention, the thin MgF₂ film is formed from asolution containing an Mg salt and BF₃ complex salt or a solutioncontaining MgF₂ sol which is applied to a substrate. If the solution isof the types of low-boiling solvent, it affords a uniform MgF₂ filmafter drying at room temperature. However, if the solution is of thetypes of high-boiling solvent, or if it is desirable to increase thestrength of the film, the substrate may be heated after coating. Theheating temperature has an upper limit which is determined according tothe softening point of glass or plastics used as the substrate.

The heating temperature should be higher than 50° C., preferably in therange of 100° to 500° C.

The solution can be applied to the substrate by spin coating, dipping,spraying, roll coating or meniscus coating. Spin coating is desirablebecause of its ability to form uniform film invariably in massproduction. It will form an MgF₂ film having a thickness of 0.01 to 1μm.

The present invention places no limit on the substrate on which isformed an MgF₂ film or low-reflection film containing MgF₂. It may beselected, according to the intended use, from glass (such as soda limesilicate glass, aluminosilicate glass, borosilicate glass, lithiumaluminosilicate glass and quartz glass), transparent ceramics (such assingle crystal of corundum, magnesia and sialon), and plastics (such aspolycarbonate). In other words, the MgF₂ film or the low-reflection filmcontaining MgF₂ may be formed on the front plate or front panel of ananti-reflection glass plate, lens, CRT panel, copy board for duplicatingmachine, instrument panel, clean room glass, CRT or LCD display panel,EC display panel, etc.

The method for forming a thin MgF₂ film may be used to form a multilayerlow-reflection film containing an MgF₂ film. The multilayerlow-reflection film having the anti-reflection performance may betypically made up of two layers, three layers or four layers. Thetwo-layer low-reflection film is made up of a high-refraction layer--low-refraction layer (λ/2-λ/4 or λ/4-λ/4 optical thickness), arrangedon top of the other, with the first layer being adjacent to thesubstrate (λ represents the wavelength of the light to be kept fromreflection). The three-layer low-reflection film is made up of amedium-refraction layer--high-refraction layer--low-refraction layer(λ/4-λ/2-λ/4 optical thickness), arranged on top of the other, with thefirst layer being adjacent to the substrate. The four-layerlow-reflection film is made up of a low-refractionlayer--medium-refraction layer--high-refraction layer--low-refractionlayer (λ/4-λ/4-λ/2-λ/4 optical thickness), arranged on top of the other,with the first layer being adjacent to the substrate. In the presentinvention, it is possible to produce a multilayer low-reflection film inwhich the outermost layer or the inner low-refraction layer is the MgF₂film (n=1.38).

The multilayer low-reflection film may contain a low-refraction layerwhich is made of an MgF₂ sol incorporated with a compound of silicon,zirconium, titanium or aluminum. This compound increases the strengthand hardness of the low-refraction layer. Incidentally, the medium- andhigh-refraction layers in the multilayer low-reflection film may be madeof any material which is not specifically limited. Examples of the knownmaterials include SnO₂, ZrO₂, TiO₂, CeO₂ and ITO, which may be usedalone or in combination with one another.

In the case where the multilayer low-reflection film contains a layermade of Sb- or F-doped SnO₂ or ITO, it exhibits antistatic propertiesowing to the low resistance of the material used. In the case of alow-reflection film of double-layer structure, the high-refraction layer(adjacent to the substrate) will exhibit antistatic properties if it ismade from a colloid solution of Sb-doped SnO₂ which is incorporated withTi(C₅ H₇ O₂)_(n) ·(OR)_(m) (n=0 to 4, m=0 to 4, and n+m=4, R=alkylgroup) or a partial hydrolyzate thereof in an amount necessary for theratio of Sb-SnO₂ to TiO₂ to be from 1:9 to 9:1 by weight. The colloidsolution may be incorporated with a silicon compound for the improvementof strength.

The anti-static low-reflection film having electrical conductivity mayhave the multilayer structure of substrate/SnO₂ /MgF₂ orsubstrate/ITO/MgF₂. (Note that one of the layers is made of atransparent, electrically conductive material.) When formed on the CRTpanel, it will prevent the CRT panel from becoming charged with staticelectricity and hence prevent the CRT panel from attracting dust andgenerating an electric discharge between the CRT panel and the humanbody.

The thin film containing an MgF₂ layer will have an increased strengthand hardness if it is of multilayer structure of substrate/SnO₂ /MgF₂-SiO₂ or monolayer structure of substrate/MgF₂ -SiO₂ or substrate/MgF₂-SiO₂ -SnO₂. The latter has an advantage of being formed by a singleapplication.

Incidentally, the above-mentioned MgF₂ sol solution may be applied tothe surface of a substrate, by spraying or the like which is followed byheating, to form an anti-glare film with minute surface irregularities.In this case, the anti-glare film may be formed on an electricallyconductive layer, or the MgF₂ sol solution may be incorporated with SnO₂or In₂ O₃ to impart antistatic properties to the film.

The method of the present invention involves the reaction between an Mgsalt and a BF₃ complex salt, with the latter functioning as afluorinating agent. In the case where the Mg salt is MgCl₂, the reactionproceeds as follows to form an MgF₂ film.

    3MgCl.sub.2 +2BF.sub.3 →3MgF.sub.2 +2BCl.sub.3 ↑

The BCl₃ in the right side might partly remain in the form of oxide inthe film; but hardly affects the properties of the film. Likewise, thereactants in the left side might partly remain unreacted; but theyhardly affect the properties of the film.

As mentioned above, the present invention provides a method for formingon a substrate a durable MgF₂ film or a durable low-reflection filmcontaining an MgF₂ film. The method is simple and efficient. That is, itconsists of coating a substrate, by spraying, spin coating or dipping,with a solution containing MgF₂ formed by the reaction between an Mgsalt and a BF₃ complex salt, or with a solution containing MgF₂ sol. Inthe case where the substrate is glass, the coating of the solution maybe heated so that a more durable thin MgF₂ film is formed on thesubstrate.

The method of the present invention permits high productivity and yetneeds only a simple equipment without vacuum system. It can be used forthe coating of large substrates such as CRT panels and glass plates formass production. Thus, the present invention is of great industrialvalue.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention will be described in more detail with reference to thefollowing Examples.

EXAMPLE 1

A solution was prepared by dissolving MgCl₂ and BF₃ ·CH₃ OH complex saltin a molar ratio of 3:2, with the total amount of the two solutes in thesolution being 5 wt%.

This solution was applied by dipping to a glass plate (as a substrate),which subsequently underwent spinning at 3000 rpm on a spin coater. Thecoated glass plate was heated at 250° C. in the air for 30 minutes.Thus, there was obtained a specimen having a 960 Å thick MgF₂ layer.This specimen was tested for reflectance of light having a wavelength of360-700 nm.

EXAMPLE 2

The same procedure as in Example 1 was repeated except that the heatingtemperature was changed to 150° C. Thus there was obtained a specimenhaving a 1150 Å thick MgF₂ layer. This specimen was tested in the samemanner as in Example 1.

EXAMPLE 3

The same procedure as in Example 1 was repeated except that the solventwas replaced by a 1:1 mixture of methanol and butanol (by volume). Thusthere was obtained a specimen having a 1000 Å thick MgF₂ layer. Thisspecimen was tested in the same manner as in Example 1.

EXAMPLE 4

The same procedure as in Example 1 was repeated except that the speed ofthe spin coater was changed to 300 rpm. Thus there was obtained aspecimen having a 1200 Å thick MgF₂ layer. This specimen was tested inthe same manner as in Example 1.

EXAMPLE 5

In a 1:1 mixture of methanol and water (by volume) were dissolved 2 wt%of SnCl₄ and 0.2 wt% of SbCl₃. This solution was applied by dipping to aglass plate, which subsequently underwent spinning at 3000 rpm on a spincoater. The coated glass plate was heated at 400° C. in the air for 30minutes. Thus there was formed a 1100 Å thick SnO₂ layer on the glassplate. On the SnO₂ layer was formed a 960 Å thick MgF₂ layer in the samemanner as in Example 1. The resulting specimen was tested in the samemanner as in Example 1.

EXAMPLE 6

In 100 g of ethanol, 3 g of H₂ O, 0.05 mol of MgCl₂ ·6H₂ O, and 0.033mol of BF₃.ethanol complex salt were dissolved. The solution wasrefluxed at 85° C. for 1 hour to give an MgF₂ sol. To the solutioncontaining MgF₂ sol, an ethanol solution of silicon ethoxide was addedso that the resulting solution contains 3 wt% of solids in the form ofoxides and also contains MgF₂ and SiO₂ in a ratio of 3:7.

To this solution, an ethanol solution of Zr(C₅ H₇ O₂)₂.(C₄ H₉ O)₂ in anamount equivalent (in terms of ZrO₂) to 10 wt% of SiO₂ was furtheradded. The resulting solution was applied to a glass plate substrate byspin coating at 3000 rpm. The coated glass plate was heated at 200° C.in air for 30 minutes. Thus there was formed on the glass plate a 950 Åthick MgF₂ layer containing ZrO₂ and SiO₂. The thus obtained specimenwas tested for reflectance of light having a wavelength of 360-700 nm.

COMPARATIVE EXAMPLE 1

The same glass plate substrate as used in Example 1, but untreated, wastested as such in the same manner as in Example 1.

The reflectance measured at a wavelength of 520 nm with regard to inExamples 1 to 5 and Comparative Example 1 are shown in Table 1 below.

                  TABLE 1                                                         ______________________________________                                        Example No.      Reflectance (%)                                              ______________________________________                                        Example 1        1.4                                                          Example 2        2.0                                                          Example 3        1.5                                                          Example 4        1.9                                                          Example 5        1.2                                                          Example 6        1.8                                                          Comparative Example 1                                                                          4.5                                                          ______________________________________                                    

EXAMPLE 7

In 100 g of ethanol, 0.05 mol of MgCl₂ ·6H₂ O and 0.033 mol of BF₃·ethanol complex salt were dissolved. The solution was reacted in aflask with a refluxing cooler at 85° C. for 1 hour to give an MgF₂ sol,followed by cooling to room temperature. The resulting solution wasapplied to a glass plate substrate by spin coating at 3000 rpm. Thecoated glass plate was heated at 200° C. in air for 30 minutes. Thusthere was formed on the glass plate a 950 Å thick MgF₂ layer. The thusobtained specimen was tested for single-sided reflectance of light at awavelength of 360-700 nm, surface resistance and strength.

EXAMPLE 8

The same procedure as in Example 7 was repeated except that the heatingtemperature was changed to 150° C. Thus there was obtained a specimenhaving a 1150 Å thick MgF₂ layer. This specimen was tested in the samemanner as in Example 7.

EXAMPLE 9

The same procedure as in Example 7 was repeated except that the glassplate was coated with a mixture of the reaction solution of Example 7(20 g) and an ethanol solution (20 g) containing 3 wt% of Si(OC₂ H₅)₄ interms of SiO₂. Thus there was obtained a specimen having a 1050 Å thickMgF₂ -SiO₂ layer. This specimen was tested in the same manner as inExample 7.

EXAMPLE 10

The same procedure as in Example 7 was repeated except that the glassplate was coated with a mixture of the reaction solution of Example 7(10 g), an ethanol solution (10 g) containing 3 wt% of Si(OC₂ H₅)₄ interms of SiO₂, and an ethanol solution (10 g) containing 3 wt% of Sb(OC₂H₅)₃ -Sn(OC₂ H₅)₄ (Sb=15 mol% of Sn) in terms of Sb-SnO₂. Thus there wasobtained a specimen having a 1000 Å thick MgF₂ -SiO₂ -Sb-SnO₂ layer.This specimen was tested in the same manner as in Example 7.

EXAMPLE 11

In a 1:1 mixture of methanol and water (by volume), SnCl₄ and SbCl₃ weredissolved such that the solution contained 3 wt% of solutes in terms ofSb-SnO₂, with the ratio of Sb to Sn being 15 mol%. The resultingsolution was applied to a glass plate substrate by spin coating at 3000rpm. The coated glass plate was heated at 400° C. in the air for 30minutes. Thus there was formed on the glass plate a 1100 Å thick Sb-SnO₂layer. On this layer was further formed a 950 Å thick MgF₂ layer in thesame manner as in Example 7. Thus there was obtained a specimen having aglass/Sb-SnO₂ /MgF₂ structure in the same manner as in Example 7. Thethus obtained specimen was tested in the same manner as in Example 7.

EXAMPLE 12

On a glass plate substrate was formed a 1100 Å thick Sb-SnO₂ layer inthe same manner as in Example 11. On the Sb-SnO₂ layer was furtherformed a 1050 Å thick MgF₂ -SiO₂ layer in the same manner as in Example9. Thus there was obtained a specimen having a glass/Sb-SnO₂ /MgF₂ -SiO₂structure in the same manner as in Example 7. The thus obtained specimenwas tested in the same manner as in Example 7.

EXAMPLE 13

The same procedure as in Example 7 was repeated except that the glassplate was coated with a mixture of the reaction solution of Example 7(10 g) and an ethanol solution (10 g) containing 3 wt% of fine SnO₂powder doped with 15 mol% of Sb (having an average particle diameter of0.02 μm). Thus there was obtained a specimen having a 1000 Å thick MgF₂-Sb-SnO₂ layer. This specimen was tested in the same manner as inExample 7.

EXAMPLE 14

The same procedure as in Example 7 was repeated except that the glassplate was coated with a mixture of the reaction solution of Example 7(10 g), an ethanol solution (10 g) containing 3 wt% of fine SnO₂ powderdoped with 15 mol% of Sb (having an average particle diameter of 0.02μm), and an ethanol solution (10 g) containing 3 wt% of Si(OC₂ H₅)₄ interms of SiO₂. Thus there was obtained a specimen having a 1050 Å thickMgF₂ -Sb-SnO₂ -SiO₂ layer. This specimen was tested in the same manneras in Example 7.

EXAMPLE 15

A first coating solution was prepared from the following components.

20 g of ethanol solution containing 3 wt% of fine SnO₂ powder doped with15 mol% of Sb (having an average particle diameter of 0.02 μm).

10 g of ethanol-water solution containing 3 wt% (in terms of TiO₂solids) of partial hydrolyzate of Ti(C₅ H7O₂)₂ ·(OC₄ H₉)₂.

2 g of ethanol-water solution containing 3 wt% (in terms of ZrO₂ solids)of partial hydrolyzate of Zr(C₅ H₇ O₂)₂.(OC₄ H₉)₂.

The coating solution was applied to a glass plate substrate by spincoating at 1500 rpm. The coated glass plate was heated at 200° C. in airfor 30 minutes. Thus there was formed on the glass plate a 1100 Å thickSb-SnO₂ -TiO₂ -ZrO₂ layer.

A second coating solution was prepared from the following components.

12 g of the reaction solution obtained in Example 7.

18 g of ethanol-water-HCl solution containing 3 wt% (in terms of SiO₂solids) of partial hydrolyzate of Si(OC₂ H₅)₄.

1.8 g of ethanol-water solution containing 3 wt% (in terms of ZrO₂solids) of partial hydrolyzate of Zr(C₅ H₇ O₂)₂ ·(OC₄ H₉)₂.

The second coating solution was applied onto the Sb-SnO₂ -TiO₂ -ZrO₂layer by spin coating at 2000 rpm, followed by heating at 200° C. in airfor 30 minutes. Thus there was formed a 1000 Å thick MgF₂ -SiO₂ -ZrO₂layer.

Thus there was obtained a specimen having a double-layer film composedof a substrate/Sb-SnO₂ -TiO₂ -ZrO₂ layer/MgF₂ -SiO₂ -ZrO₂ layerstructure. This specimen was tested in the same manner as in Example 7.

COMPARATIVE EXAMPLE 2

The same glass plate substrate as used in Example 7, but untreated, wastested as such in the same manner as in Example 7.

The results in Examples 7 to 15 and Comparative Example 2 are shown inTable 2 below.

                  TABLE 2                                                         ______________________________________                                                                Surface                                                          Reflectance  resistance                                            Example No.                                                                              (%)          (Ω/□)                                                                 Strength                                     ______________________________________                                        Example 7  0.7          .sup. 7 × 10.sup.12                                                              Δ                                      Example 8  1.0          .sup. 7 × 10.sup.12                                                              Δ                                      Example 9  1.1          .sup. 6 × 10.sup.10                                                              ∘                                Example 10 1.2          4 × 10.sup.8                                                                     ∘                                Example 11 0.6          1 × 10.sup.9                                                                     Δ                                      Example 12 1.4          8 × 10.sup.8                                                                     ∘                                Example 13 1.3          8 × 10.sup.7                                                                     Δ                                      Example 14 1.4          2 × 10.sup.8                                                                     ∘                                Example 15 0.7          8 × 10.sup.7                                                                     ∘                                Comparative                                                                              4.5          >10.sup.12                                                                             --                                           Example 1                                                                     ______________________________________                                    

The measurement method is as follows:

Reflectance: measured for light having a wavelength of 520 nm using anautomatic spectrophotometer made by Shimadzu Seisakusho Ltd.

Surface resistance: measured using a surface resistance meter. InExamples 5, 6, 10, 11-14, the surface resistance is that of the twolayers in combination.

Strength: rated depending on whether the coating film suffered visiblescratches when rubbed up to 100 times with a rubber eraser (No. 50-50made by Lion Jimuki Co., Ltd.) under a load of 1 kg.

X Peeling of a film (visible scratches on the glass plate) occurredwithin 50 times rubbing;

ΔPeeling of a film occurred in 50 to 100 time rubbings; and

◯Peeling of a film not occurred even after 100 time rubbings.

We claim:
 1. A method for forming a thin MgF₂ film on a substrate, whichcomprises coating a substrate with a liquid containing an Mg salt and aBF₃ complex salt and subsequently heating the coating to form a filmMgF₂ film.
 2. The method for forming a thin MgF₂ film according to claim1, which comprises coating a substrate with a liquid containing an MgX₂(X=a halogen element other than fluorine) and a BF₃ complex salt andsubsequently heating the coating.
 3. The method for forming a thin MgF₂film according to any of claims 1 and 2, wherein the BF₃ complex salt isat least one kind selected from BF₃ alkyl ether complex salt, BF₃ phenolcomplex salt, BF₃ alcohol complex salt and BF₃ aqueous solution complexsalt.
 4. A method for forming on a substrate a thin MgF₂ film, whichcomprises coating a substrate with a liquid containing an MgF₂ solformed by the reaction between an Mg salt and a BF₃ complex salt, andsubsequently heating the coating.
 5. The method for forming a thin MgF₂film according to claim 4, which comprises coating a substrate with aliquid containing an MgF₂ sol formed by the reaction between an Mg saltand a BF₃ complex salt and silicon compound incorporated therein, andsubsequently heating the coating.
 6. The method for forming a thin MgF₂film according to claim 4 and claim 5, which comprises coating asubstrate with a liquid containing an MgF₂ sol formed by the reactionbetween an Mg salt and a BF₃ complex salt and being incorporated with afine powder of electrically conductive oxide or a metal salt which formsa fine powder of electrically conductive oxide, and subsequently heatingthe coating.
 7. The method for forming a thin MgF₂ film according to anyof claims 4 and 5, wherein the Mg salt is at least one kind of saltsselected from halides expressed by the general formula MgX₂ (X=a halogenelement other than fluorine).
 8. The method for forming a thin MgF₂ filmaccording to claim 4, which comprises coating a substrate with a mixtureof a liquid containing an MgF₂ sol formed by the reaction between an Mgsalt and a BF₃ complex salt with a liquid containing at least one kindselected from silicon compound, zirconium compound, titanium compound,aluminum compound and tin compound, and subsequently heating thecoating.
 9. A method for forming on a substrate a low-reflection film ofmonolayer or multilayer structure, with at least one layer being a thinMgF₂ film, said process comprising coating a substrate with a liquidcontaining an Mg salt and a BF₃ complex salt and subsequently heatingthe coating.
 10. A method for forming on a substrate a low-reflectionfilm of monolayer or multilayer structure, with at least one layer beinga thin MgF₂ film, said process comprising coating a substrate with aliquid containing an MgF₂ sol formed by the reaction between an Mg saltand a BF₃ complex salt, and subsequently heating the coating.
 11. Themethod for forming on a substrate a low-reflection film according to anyof claim 9 and claim 10, wherein the BF₃ complex salt is at least onekind selected from BF₃ alkyl ether complex salt, BF₃ phenol complexsalt, BF₃ alcohol complex salt and BF₃ aqueous solution complex salt.12. The method for forming on a substrate a low-reflection film ofmonolayer or multilayer structure, with at least one layer being a thinMgF₂ film, according to claim 9, wherein the thin MgF₂ film is formed bycoating a substrate with a liquid containing an MgX₂ salt (X=a halogenelement other than fluorine) and a BF₃ complex salt, and subsequentlyheating the coating.
 13. The method for forming on a substrate alow-reflection film of monolayer or multilayer structure, with at leastone layer being a thin MgF₂ film, according to claim 10, wherein thethin MgF₂ film is formed by coating a substrate with a liquid containingan MgF₂ sol formed by the reaction between an Mg salt and a BF₃ complexsalt, and subsequently heating the coating.
 14. The method for formingon a substrate a low-reflection film of monolayer or multilayerstructure, with at least one layer being an MgF₂ -containing film,according to claim 9, wherein the MgF₂ -containing film is formed bycoating a substrate with a mixture of a liquid containing an Mg salt anda BF₃ complex salt with a liquid containing at least one kind selectedfrom silicon compound, zirconium compound, titanium compound, aluminumcompound and tin compound, and subsequently heating the coating.
 15. Themethod forming on a substrate a low-reflection film of monolayer ormultilayer structure, with at least one layer being an MgF₂ -containingfilm, according to claim 10, wherein the MgF₂ -containing film is formedby coating a substrate with a mixture of a liquid containing an MgF₂ solformed by the reaction between an Mg salt and a BF₃ complex salt and aliquid containing at least one kind selected from silicon compound,zirconium compound, titanium compound, aluminum compound and tincompound, and subsequently heating the coating.
 16. The method forforming on a substrate an electroconductive low-reflection film of atleast two layer structure, according to claim 9, which comprises forminga transparent electroconductive film on a substrate, coating thereon aliquid containing an MgX (X=a halogen element other than fluorine) saltand a BF₃ complex salt, and subsequently heating the coating to form athin MgF₂ film.
 17. The method for forming on a substrate anelectroconductive low-reflection film of at least two layer structure,according to claim 10, which comprises forming a transparentelectroconductive film on a substrate, coating thereon a liquidcontaining an MgF₂ sol formed by the reaction between an Mg salt and aBF₃ complex salt, and subsequently heating the coating to form a thinMgF₂ film.